This patch contains SCST SGV cache. SGV cache is a memory management subsystem in SCST. One can call it a "memory pool", but Linux kernel already have mempool interface, which serves different purposes. SGV cache provides to SCST core, target drivers and backend dev handlers facilities to allocate and build SG vectors for data buffers. The main feature of it is that it doesn't free to the system each vector, which is not used anymore, but keeps it for a while to let it be reused by the next consecutive command to reduce command processing latency and, hence, improve performance. The freed SG vectors are kept by SGV cache either for some predefined time, or until the system needs more memory and asks to free some using the set_shrinker() interface. Also the SGV cache allows to:

- Cluster pages together to minimize number of SG entries in the vector and improve the performance of handling the SG vector.

- Prevent each initiator or all initiators altogether to allocate too much memory and effectively DoS the target. Consider 10 initiators, which can have access to 10 devices each. Any of then can queue up to 64 commands, each can transfer up to 1MB of data. So, all of them in a peak can allocate up to 10*10*64 = ~6.5GB of memory for data buffers. This amount must be limited somehow and SGV cache performs this function. This feature was implemented after people reported about such DoS'es, when there are many fast initiators and a slow target.

From implementation POV SGV cache is a simple extension of kmem cache. Each SGV cache, called pool, (struct sgv_pool) has SGV_POOL_ELEMENTS (11 currently) of kmem caches. Each of those kmem caches keeps SGV pool objects (struct sgv_pool_obj) corresponding to SG vectors with size of order X pages. For instance, request to allocate 4 pages will be served from kmem cache[2] (order 2). If then request to allocate 11KB comes, the same SG vector with 4 pages will be reused (see below).

When a request to allocate new SG vector comes, sgv_pool_alloc() via sgv_pool_cached_get() checks if there is already cached vector with that order. If yes, then that vector will be reused and its length, if necessary, will be modified to match the requested size. In the above example request for 11KB, 4 pages vector will be reused and modified using trans_tbl to contain 3 pages and the last entry will be modified to contain the requested length - 2*PAGE_SIZE. If there is no cached object, then a new sgv_pool_obj will be allocated from the corresponding kmem cache, chosen by order of number of requested pages. Then that vector will be filled by pages and returned.

Freed sgv_pool_obj objects are freed to the system either by apit_pool work or in sgv_pool_cached_shrinker() called by system, when it's asking for memory.